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Geosyntec i C11151111,111Ch 2.4 Nature and Extent of Sulfolane Impacts Historical groundwater monitoring data and preliminary results from the ongoing supplemental characterization suggest that sulfolane in the eastern portion of the Site has migrated vertically downward to the well-graded sand and gravel layer overlying the Corcoran Clay,and then laterally through this transmissive layer towards the water supply wells. Consistent with the geologic logs, preliminary results from ongoing supplemental characterization suggest that the primary sulfolane transport pathway in the eastern portion of the Site near the BSW and the PSW is within the approximately 20 to 30-foot-thick highly transmissive well-graded sand and gravel layer overlying the Corcoran Clay (Figure 3). Preliminary grab-groundwater and monitoring well results from Locations 1 through 4 of the supplemental characterization work (Figure 3) are available and indicate that sulfolane concentrations in the transmissive layer above the Corcoran Clay in vicinity of the PSW and BSW range from 57 to 200 micrograms per liter (µg/L) and 230 to 730 µg/L, respectively. Sulfolane concentrations at Location 1, which is closest to LWD-10, were 32 ug/L in the groundwater grab sample but were non-detect (<10 µg/L) in the monitoring well sample. The monitoring well sample was collected at a time when LWD-9 and 10 were not pumping. 2.5 Preliminary Evaluation of Potential Remedial Alternatives Concurrent with the ongoing supplemental characterization, Geosyntec has undertaken a preliminary evaluation of potential remedial alternatives to prevent migration of sulfolane from the Site towards the Simplot and LWD supply wells. The potential remedial alternatives considered included ISB using biosparging or passive-diffusion barriers and ex-situ biological treatment using various types of bioreactors and treated water discharge options. Considering timing, implementability and likely effectiveness, and given the recent repeatable observations of sulfolane degradation (presumed to be aerobic biodegradation) in un-preserved BSW water samples, ISB using biosparging was considered to be a feasible remediation approach to degrade and prevent further Sulfolane migration towards the supply wells. As indicated above, Sulfolane in unpreserved BSW samples completely degraded during transit to the analytical laboratory. In addition, sulfolane spiked into the same BSW sample continued to exhibit complete degradation within 2 hours, confirming previous evidence of biodegradation. Similar sulfolane degradation was not observed in groundwater collected from the PSW, suggesting that either the naturally-occurring microorganisms in the subsurface at the PSW have not yet evolved to biodegrade sulfolane,the right nutrients are not available for the microorganisms to degrade sulfolane, or that the observed degradation in the BSW groundwater is related to other degradation mechanisms (anaerobic or abiotic). To confirm the degradation mechanism, focused microcosm studies are being conducted using aquifer material and groundwater from the supplemental characterization locations near the BSW (Location 3), the PSW (Location 2) and LWD-10 (Location 1), concurrent with the development of this Work Plan. The objectives of microcosm studies are to confirm the observed sulfolane In-Situ Bioremediation Pilot Test Work Plan—GSH Lathrop 4